The present application claims priority to Italian Patent Application No. MI 2000A 002614, filed Dec. 4, 2000.
The present invention relates to a process for the production of expanded polypropylene and end-products based on expanded polypropylene.
Whereas polymers such as polyethylene or polystyrene do not have any particular problems when subjected to an expansion process by the use of chemical or physical agents, polypropylene is a polymer having various characteristics which do not allow easy expansion. It is, in fact, characterized, in the molten state and at its typical rheological temperature, by a fluidity which is such that the action of gases used for expanding the cells, causes the polypropylene structure to collapse.
The production of expanded polypropylene end-products, on the other hand, is particularly interesting due to the low cost of polypropylene and to its good mechanical and thermal resistance.
Processes are known for the production of expanded polypropylene which, by means of an extrusion process, chemically expand this polymer with the use of chemical expanding agents, obtaining expanded products with specific weights higher than 0.6 kg/dm3.
Furthermore, chemical expanding agents, used with percentages equal to 2-6% with respect to the polypropylene, are very expensive. There are also methods in which the expansion is obtained by the use of physical agents such as CO2, which are injected into the molten polymer. SUMMARY OF THE INVENTION
The objective of the present invention is therefore to produce expanded polypropylene end-products which can be used in various fields such as, for example, the car industry (internal door linings), for food packaging, fruit containers, etc.
In particular, the objective of the present invention is to produce end-products consisting of sheets or plates, of varying dimensions, using polypropylene, whose mass is expanded with closed micro-cells by the use of CO2, having a specific weight ranging from 0.5 kg/dm3 to 0.3 kg/dm3, preferably a specific weight equal to about 0.35 kg/dm3.
This objective according to the present invention is surprisingly achieved by means of a process comprising the following phases:
(a) the feeding and melting of polypropylene to obtain a molten polymer;
(b) addition to the molten polymer of possible additives;
(c) mixing of the molten polymer with the additives;
(d) introduction of the expanding agent CO2 into the molten polymer;
(e) extrusion of the molten polymer, expanding agent and possible additives to produce an expanded polypropylene;
said process being characterized in that the polypropylene fed and melted in phase a) consists of a polypropylene which contains a quantity ranging from 15 to 25% of high melt strength polypropylene and in that the temperature is lowered to a value of about 160xc2x0 C. immediately after the introduction of the CO2 gas into the molten polymer.
In particular, the introduction of the expanding agent CO2 into the molten polymer, i.e. phase d), can be effected by injecting liquid gas under pressure into the molten mass or by the injection of expanded gas under vacuum.
The high melt strength polypropylene can be partly or completely substituted with polyethylene.
The additives can consist of mineral fillers, such as CaCO3, talc or others.
A further object of the present invention relates to end-products based on expanded polypropylene obtainable with the process according to the present invention.
Another object of the present invention also relates to the use of expanded polypropylene obtainable with the process according to the present invention for the production of end-products, in particular of end-products in the car industry, such as internal door linings, and in the field of food packaging, such as fruit containers.
The main advantage of the process according to the present invention lies in the production of expanded polypropylene end-products having excellent mechanical and thermal resistance, and at the same time at a particularly low cost.
A further advantage consists in the fact that the expanded polypropylene end-products, in particular plates, obtainable by the process according to the present invention, have a smooth breakage and are therefore particularly useful in the car industry, especially for internal door linings, thanks to the safe properties of the material.
Another advantage of the process according to the present invention is linked to the use of CO2. CO2, in fact, can be handled without danger, it is ecologically more convenient with respect to other gases as it is a natural re-used resource, is produced without any kind of burning and at a low cost; CO2 furthermore is the most suitable gas for a non-violent expansion.